Npower, one of Britain’s largest energy companies, is expected to announce up to 2,500 job cuts in the UK and overseas this week after being blighted by an exodus of customers, turbulent world energy markets and embarrassing customer billing problems.

The German-owned business, which is one of Britain’s “big six” energy suppliers, will tell staff thousands of jobs are to be axed following a series of poor financial results.

Details of the job losses could be revealed as soon as Tuesday when parent group RWE releases its latest financial results.

Npower employs 7,500 people directly in the UK but also has a sizable workforce overseas. In recent years it has closed UK offices and outsourced customer service work to India. It is thought a proportion of the 2,500 job cuts will come from such offshore operations.

On Sunday, the company would not comment on leaked news about job losses, but dramatic changes to the business have been anticipated for some time.

RWE last month slashed dividends after posting a loss of €200m (£155m) for 2015.

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In November, Bernhard Guenther, RWE’s chief financial officer, hinted that the energy group could even end up selling its troubled npower unit, as the wider group struggled to cope with financial problems on several fronts.

He said npower was unlikely to return to profit until 2017 at the earliest. RWE shareholders are understood to have questioned whether the business is worth keeping.

As well as persistently low wholesale electricity prices, npower has also had to contend with the threat of increased competition from newcomers such as Ovo and First Utility.

Unions reacted angrily to job cut plans being leaked to the media before staff had been informed.

“We will be demanding urgent answers and assurances from npower over the coming days as we seek to protect as many jobs as possible,” said Kevin Coyne, Unite’s national organiser for the energy industry.

A spokesman for the GMB union said npower was the third major energy firm to announce significant cuts after Centrica and EDF.

“Many hardworking staff have already been outsourced and so any further job losses for directly and indirectly employed staff would be another kick in the teeth for deindustrialised communities,” he said.

“It also makes a mockery of George Osborne’s rhetoric about a northern powerhouse if an energy company can’t maintain jobs in the north-east.”

Shares in RWE, which is listed on the Frankfurt stock exchange, have more than halved in the last year. Its chief executive, Peter Terium, noted that investors had been particularly frustrated by embarrassing billing mistakes at npower.

Some British households had reportedly received multiple bills for varying amounts, not matched to meter readings.

In other instances the names on bills have been incorrect, or bills have been arriving late or not at all. Npower’s financial performance was also hit last year by lower UK energy consumption and higher grid fees.

Last August, RWE said npower’s profit for the first six months of 2015 had fallen by almost two-thirds to £38m. Npower boss Paul Massara quit later that month.

In a trading update in November, RWE said npower had lost about 200,000 customers in Britain since the start of 2015.